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Thread: Engine tach pulse division

  1. #11
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    Quote Originally Posted by Chuck(G) View Post
    According to the Toptop site, you can set the thing up:
    I can't figure out the rate adjustments at all. 1 is off by thousands of revs a minute, 2-7are somewhere in between, but closer, 8 is closest, but that doesn't really make sense on a 6 cylinder engine, and 9-99 is still all over the place.
    Plus, this has no decimal point. These instructions don't make a lot of sense. That's partly why I don't mind having something placed before the tachometer to "condition" the signal I guess.

    @dwight, the MCU has an internal crystal, so that won't work. It is however a a solid-state system. A DuraSpark II module generates the tach pulse. Peak side of the pulse appears to be around 12v.
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  2. #12
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    Someone suggested switching to frequency counter mode and setting the rate to 6. This just removes a digit and isn't all that accurate.



    I went back to RPM mode and reset the rate to 6 and it's still not all that great. The total RPM can vary as much as 300 in either direction which is unusable for tuning. I know these are much more accurate than that.




    This is one operating with a 1:1 ratio on a small gas engine. My screen is refreshing six times faster than that (because its being pulsed six times as much) and with the above variance makes it impossible to read.
    https://www.youtube.com/watch?v=Mte4v32qrO4

    Someone suggested something like a 4017B or CD4022b with an optocoupled input from the tach. I've never worked with either of these chips though.
    Last edited by NeXT; June 11th, 2019 at 05:55 PM.
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  3. #13

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    Can you put a scope on the Tach output of our car? It may not be anything like you think.
    Also, it is a 4018 you want, not the other two.
    See http://www.bitsavers.org/components/..._Data_Book.pdf
    Page 7-62

    I would think you'd set the number to 3 or 30. A six cylinder engine ( 4 cycle ) only puts out 3 ignitions per rotation, no 6. 6 would be a two cycle. On a small engine, it fires the sparkplug once per revolution but only one of the two times causes an ignition, the other doesn't do anything because the gas is already used up.
    Dwight
    Last edited by Dwight Elvey; June 11th, 2019 at 07:03 PM.

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    Quote Originally Posted by Dwight Elvey View Post
    I realize, many older capacitive discharge used the mechanical points. I consider that to still be points( although solid state firing ).
    Newer cars no longer use points. There are variations, depending on how new. Some have magnetic pickups in a distributor, some have multiple coils, some have crankshaft pickups and some sync these with cam shaft pickups ( some don't ). These that don't have points I'm lumping together as solid state.
    If it doesn't have points, it has to be some form of solid state.
    This doesn't; it uses a hall effect sensor in the distributor. It has a pretty weak spark; I always replace them with MSD 6. There's a way to adapt a larger distributor cap, and a cap with studs instead of sockets. It makes for a very reliable ignition

  5. #15
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    Quote Originally Posted by Dwight Elvey View Post
    Can you put a scope on the Tach output of our car? It may not be anything like you think.
    Here's a waveform for you.



    0v and 14v are pretty clear. The only thing to note is the 35v transient spike from the coil firing. It's weak enough that everything else in the car doesn't blow up from it so don't revolve around it too much as the rest of the engine operates normally with it however it would explain why a meter designed for a max 24v input would kill a resistor. Perhaps I do need to decouple the tach signal in order to better condition it into something more resembling a square wave?

    I would think you'd set the number to 3 or 30. A six cylinder engine ( 4 cycle ) only puts out 3 ignitions per rotation, no 6. 6 would be a two cycle. On a small engine, it fires the sparkplug once per revolution but only one of the two times causes an ignition, the other doesn't do anything because the gas is already used up.
    Dwight
    Question: IF at a division of 6 is already somewhat close, wouldn't a division of 3 result in a larger reading?
    Last edited by NeXT; June 11th, 2019 at 08:01 PM.
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  6. #16

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    What size resistor are you using? I find it hard to believe any reasonable size resistor could be damaged with a small 35V spike.
    On the scope, I don't know what the sweep scale is or about what RPM it is running at?
    Dwight

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    Also it would be nice to see the waveform with a resistor in parallel to the scope.

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    The original was some itty-bitty SMD thing. It didn't clear the board or really burn up but just read infinite and refused to pass anything through it. I replaced it with a larger 2K 1/8 watt metal film resistor which is still going strong.

    Quote Originally Posted by KC9UDX View Post
    Also it would be nice to see the waveform with a resistor in parallel to the scope.
    How about I just post a photo of the back of the tach so you can see how the input circuit works? Note that R16 was the SMD that had to be replaced.

    Last edited by NeXT; June 11th, 2019 at 09:21 PM.
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  9. #19
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    Well what I was interested in was the amount of current available in the spike.

  10. #20
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    Im interested in what car it is

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